Facebook Inc. took eight years to stage one of the most anticipated initial public offerings ever. The anticlimax came Friday, as Wall Street bankers struggled to prevent the newly minted stock from ending its first day with a loss.

The stock had been widely predicted to soar on its first day. Instead, up until the closing moments of the trading session, Facebook's underwriters battled to keep the stock from slipping below its offering price of $38 a share. Such a stumble would have been a significant embarrassment, particularly...